Blocking a user does mean that the blocked user cannot send you any messeges anymore, rather if he wants to send
you a message this is disabled (a friendly message will appear on the blocked users screen saying that you prefer not to receive any messages at this time).
After the massive system rebuilding in May 2005, replying for blocked users to your messages is not possible anymore, hence
the block users cannot initiate any contact with you anymore.

Example: a user is sending you messages you do not want to receive. Click the block-image.
From then that user is not able to send messages to you anymore.

Alternatives

Instead of blocking a user, you can also choose to remove this user from your online or new lists;
in that case the user does not show up there anymore. That user is still able to send you messages though.
You can also use the No Thanks option; it appears with new messages. If you use that,
the addressee gets an autoreply message saying you are not interested.

We may suggest to use the No Thanks- Autoreply first, and if the user keeps persisting, use the block option
as a last resort.

Blocking a user is the only way you are sure that user cannot send you any messages;
Unblocking a user can be done in Your Contacts.